The invention resides in a method of operating an internal combustion engine wherein exhaust gas is recirculated from an exhaust pipe to an intake duct of the engine in order to reduce engine emissions and the exhaust gas recirculation rate is controlled by an exhaust gas recirculation valve whose flow cross-section is controlled by a control unit depending on the engine operating point, and to an internal combustion engine suitable for performing the method.
The exhaust gas recirculation is an effective means for reducing exhaust emissions of an internal combustion engine. By admixing already burnt exhaust gases to be fresh gas admitted to the engine, it is possible to reduce particularly the temperature-dependent NO.sub.x emissions of the engine. Generally, exhaust gas is recirculated from an exhaust gas duct of the engine back to an engine intake duct. The recirculation rate for the exhaust gas, that is, the amount of exhaust gas admixed to the fresh air flow in the intake duct is controlled by a control unit depending on engine operating points recorded in a performance graph. An increase of the exhaust gas recirculation rate for further reduction of the nitrogen oxide emissions however is limited to degrees which depend on the respective engine operating points since, above a certain exhaust gas recirculation rate carbon and particle emissions increase, the fuel consumption increases or the engine running smoothness suffers.
The exhaust gas recirculation rate control requires that the recirculation conditions are constantly monitored during the operation of the engine. With simple control procedures for controlling the exhaust gas volume recirculated such as an engine operating condition-dependent control of an adjustable exhaust gas recirculation valve the exhaust gas emissions can be lowered only unsatisfactorily since, at any particular operating point, the recirculation rate must be lower than the optimum desired value in order to avoid the otherwise frequent transgression into an area where excessive carbon and correspondingly high emissions are generated.
EP 0 574 614 A1 discloses a method for controlling the exhaust gas recirculation rate which measures the pressure drop in a venturi nozzle arranged in the exhaust recirculation passage and determines from the pressure drop the actual recirculation gas flow volume. However, since the exhaust gas recirculation flow through the exhaust gas recirculation passage depends on the pressure drop between the exhaust gas pipe and the air intake duct of the internal combustion engine, the pressure loss in the venturi nozzle reduces the pressure drop available for the recirculation of the exhaust gas. In this way, the possible exhaust gas recirculation rate is reduced so that for many engine operating points the exhaust gas emissions cannot be optimally reduced. In addition, the determination of the exhaust gas flow volume in the exhaust gas recirculation pipe by way of measuring the differential pressure is very inaccurate since measurements by way of a venturi nozzle require laminar flow conditions which can practically not be achieved in the exhaust gas flow of internal combustion engines. The more or less turbulent exhaust gas flows of the various internal combustion engines, particularly of supercharged internal engines, prevent an effective control of the exhaust gas recirculation rate with the conventional methods.
DE 43 37 313 C1 proposes to measure the absolute pressure and the temperature in the exhaust gas pipe of an internal combustion engine in order to determine the recirculated exhaust gas volume. For determining the exhaust gas recirculation rate in addition to the static pressure also the back-up pressure of the exhaust gas is needed. With the shown methods however, an accurate determination of the exhaust gas recirculation rate which is absolutely needed for an optimal engine control is impossible since the necessary temperature and pressure sensors are exposed to very high temperatures. Furthermore, a high carbon particle content of the exhaust gas results in deposit formation on the sensors which increases with increasing engine operating times so that the measurements become increasingly inaccurate and finally lead to increased exhaust emissions.
It is the object of the present invention to provide for maximal emission reductions of internal combustion engines by accurately determining and controlling the exhaust gas recirculation rate.